The present invention relates to solar cells and in particular to light transmitting, electrically conductive layers in intimate electrical contact with solar cells.
The efficiency of a solar device in converting light incident upon the device into electrical energy is controlled in part by the percentage of light transmitted to the amorphous silicon portion. Light which is reflected or absorbed within the top conductive layer, used for electrical interconnection of the device, is unavailable for conversion into electrical energy by the amorphous silicon portion. Additionally, the efficiency of the device is dependent upon the ability of the transparent conductive layer to conduct electrons either away from or to the amorphous silicon region. Conductivity is particularly important in high output solar devices or large area devices.
Generally, efficient transmission of light and efficient conduction of electricity by the top conductive layer are conflicting goals. For example, thick metallic conductors (i.e. greater than 50 nm) are opaque to light. Very thin films of metal, however, are not opaque. A few metals (notably silver and aluminum) have low light absorption as thin films. However, these same metals are generally highly reflective, even as very thin films, thus resulting in low light transmission.
One approach of the prior art has been the utilization of metal oxides which are both transparent and have a rather modest electrical conductivity. "Transparent," with respect to metal oxides, means that some light is transmitted through the material. The transparent conductive oxides require excessively thick layers or excessively high deposition and/or annealing temperatures to achieve desirable levels of electrical conductance; however, the excessively thick layers of transparent conductive oxides cause the device to suffer from reduced transmission of light because of increased optical absorption.
The combination of transparent conductive oxides and metallic conductors has been suggested. Madan in U.S. patent application Ser. No. 061/676,047 filed Nov. 29, 1984, discloses amorphous silicon semiconductors contacting a thin metallic layer with one overlying antireflection layer approximately 55 to 60 nm thick on top of the metallic layer. The Madan antireflection scheme becomes less efficient when the encapsulant necessary for mechanical and chemical protection of the device is applied over the antireflection layer. Specifically, the reflectance increases by 15 to 25% after a typical encapsulation. The Berman et al. U.S. Pat. No. 4,663,495, discloses transparent electrodes of high reflectance metals and metal oxides laid up in symmetrical stacks. The Okaniwa et al. U.S. Pat. No. 4,697,041, suggests the use of a laminate such as Ti02/Ag/Ti02 as a transparent electrode.